The code division multiple access (CDMA) technology has been proposed in recent years for multiple access in mobile communication systems. The CDMA technology is adapted to multiplex signals that are modulated by the spread spectrum on a radio transmission line (to be referred to as channel hereinafter) defined by using a code specific to each user of the same radio frequency.
Roughly classifying, two methods are known as the spread spectrum; direct sequence (DS) and frequency hopping (FH), of which the DS is designed to modulate the carrier wave by means of a PN (pseud noise) signal code that shows a bit rate greater than the information rate to be used for communication and transmit the signal with a spread frequency band. The receiving side takes out only the signal showing a bit string same as the PN signal code from the received radio wave and demodulates it to obtain the original information. With this arrangement, the interference and noise components that are overlaid on the proper signal on the transmission line can be removed by the technique of spread spectrum at the time of demodulation.
A mobile terminal, which is designed for use with the CDMA technology and may typically be a portable phone, is adapted to select a channel to be used for transmission of information before making a call (for signal transmission) or receiving a call (for signal reception) and transmitting/receiving messages and start capturing the selected channel. Upon capturing the selected channel, it obtains information on the configuration of the captured channel and the timing of operation and synchronizes the timing of operation of itself and that of the captured channel.
FIG. 1 of the accompanying drawing is a flow chart illustrating the sequence of operation of capturing a channel by means of a known channel capturing method.
Referring to the flow chart of FIG. 1, as power is supplied to the mobile terminal, it initializes one of the existing channels A and B, which may be selected arbitrarily. Subsequently, it tries to capture the initialized channel. Therefore, if, for example, the channel A is initialized, the mobile terminal 1 tries to capture the channel A.
If the mobile terminal 1 fails to capture the initialized channel A, it selects a channel that is not the initialized channel, or the channel B, (ST11) and tries to capture the channel B (ST12).
After trying to capture the selected channel B, the processing operation of the mobile terminal 1 proceeds to Step ST13, where it determines if it has succeeded in capturing the selected channel B or not by referring to the ID number of the base station and certain parameters such as the field strength for signal reception.
If it is determined in Step ST13 that the channel B has not been captured, the effort for capturing the channel is regarded to have met with failure (ST14).
If, on the other hand, it is determined in Step ST13 that the channel B has been captured by the mobile terminal so as to effectively receive messages by way of the channel specified by the channel list message that is addressed to it, the mobile terminal moves into a standby state of being ready for receiving a signal without falling into a condition when a system was lost (ST15).
However, the mobile terminal may fail in capturing a channel A showing a high field strength but succeed in capturing a channel showing a low field strength.
Assume here, for instance, that the channel A shows a high field strength while the channel B shows a low field strength and the mobile terminal fails in capturing the channel A which it has initialized so that it selects the channel B in Step ST11 and succeeds in capturing the channel B. Then, it is no longer possible for the mobile terminal to effectively receive any message by way of the channel specified in the channel list message addressed to it and the mobile terminal falls in a condition when a system was lost because it captures the channel B showing a low field strength (ST16).
Thus, the prior art is accompanied by the problem that, once the mobile terminal falls in a condition when a system was lost, it keeps on trying to capture the specified channel in a condition when a system was lost as shown in FIG. 1. The net result is that the mobile terminal no longer effectively receive any message and remains in a condition when a system was lost.